The contribution of De Facto Regimes to the Formation of Customary International Law

A de facto regime is defined as an entity “that exercises at least some effective political authority over a territory within a State”, “without being recognized as independent State or government of an already existing State”. De facto regimes (DFRs) possess rights and obligations in international law-making as far as States allow them to act in this domain, for example in multilateral status settlement negotiations, in territorial or maritime boundary delimitation, commercial law matters etc. The first part of the paper illustrates that DFRs could participate in the formation of sources of international law other than customary international law (CIL), i.e. international conventions, acts of international organizations and especially of international tribunals. Those acts can indirectly build CIL. Beyond this indirect role, the second part of the paper examines whether DFRs may directly contribute to the formation of CIL through their “practice”, which is not necessarily, but might be considered as State practice. The paper argues that the ILC’s conclusion about the relative relevance of the conduct of non-state actors when assessing the practice of States and international organizations is a fortiori applicable to DFRs by reason of their state-like character, i.e. their effective exercise of governmental functions.